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7591 Bass Amp - Need More Power
7591 Bass Amp - Need More Power
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Old 15th August 2018, 01:25 PM   #1
ITPhoenix is offline ITPhoenix  United States
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Default 7591 Bass Amp - Need More Power

This is out of a Hammond H-100. The bass signal is "derived" by combining the left and right channels, then filtering for the bass section. The tubes are new, the DC output on the recto tubes does not dive under load, all electro caps have been replaced, the speaker is good, and it sounds good, except the lower end seems weak in terms of power.


Is there anything to be checked, or some modification be made that won't ruin anything?


Thanks.
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Old 15th August 2018, 02:08 PM   #2
Yvesm is offline Yvesm  France
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Did you measure the output power ?
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Old 15th August 2018, 02:19 PM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Should be good for up to 40Watts. The bass response is down to the amount of iron in the output transformer and the speaker/cabinet type.


Just a thought, I had a Fender Twin Reverb, 1965 model with poor bass response, that was shorted turns in the output transformer, causing lack of flux and poor bass response.
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Last edited by JonSnell Electronic; 15th August 2018 at 02:35 PM. Reason: More information added.
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Old 15th August 2018, 03:08 PM   #4
ITPhoenix is offline ITPhoenix  United States
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Originally Posted by Yvesm View Post
Did you measure the output power ?
4 Ohm resistive load: 40V 1.6A

4 Ohm Speaker: 60V 1.5A

I increased the input signal significantly with no effect. I can do this easily since I replaced all the preamps with opamps. It's as if there is a limiter in there.

Thank you for the prudent idea.
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Old 15th August 2018, 03:27 PM   #5
Yvesm is offline Yvesm  France
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Hi,
There is something wrong somewhere !
40V across a 4 Ohm resistor should produce a current of 10A and a power of 400W.
How hot is the resistor ?
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Old 15th August 2018, 03:35 PM   #6
ITPhoenix is offline ITPhoenix  United States
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Originally Posted by JonSnell Electronic View Post
Should be good for up to 40Watts. The bass response is down to the amount of iron in the output transformer and the speaker/cabinet type.

Just a thought, I had a Fender Twin Reverb, 1965 model with poor bass response, that was shorted turns in the output transformer, causing lack of flux and poor bass response.

According to the manual, total power for L, R, and Bass is 60W E.I.A. (The EIA rating, established by The Electronic Industries Association, reflects the power output for a single channel driven at mid-band – typically 1 kHz – with 1% THD clipping. This standard inflates the amplifier’s power points to 10 to 20% higher than the FTC ratings.)

Perhaps I'm expecting too much. There is no cabinet--it's open air. The output transformer is 50 years old, and there's no data given for resistance for my particular model. The diagram indicates an 8 Ohm speaker--mine has a 4 Ohm. In fact, I have been unable to locate the correct diagram for my unit, except that I know it' the last of the Mahicans.

I have the big bottle 7591s, so perhaps an OT with different characteristics could boost the output without smoking something. Incidentally, I did zero out the balance pot so there was no DC on the OT input. It was not off much at all.

Thank you for your ideas.
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Old 15th August 2018, 03:45 PM   #7
ITPhoenix is offline ITPhoenix  United States
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Hi,
There is something wrong somewhere !
40V across a 4 Ohm resistor should produce a current of 10A and a power of 400W.
How hot is the resistor ?
I will check it again. I'm working with a $30 Radio Shack DMM that's 15 years old, and perhaps I messed up the connections.

I just replaced the speaker with a 4 Ohm, 20W ceramic and ran max output for 30 seconds. I got warm but 10A would have been untouchable.

It seems that perhaps nothing is wrong.
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Old 15th August 2018, 03:54 PM   #8
mondogenerator is offline mondogenerator  England
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I'm no valve amp.expert but I recall reading that using a 4 Ohm load on 8 Ohm tap will shift the high pass frequency up (DC blocking and infra sound filter). I guess it follows that the high end roll off would be altered too.

That may account for your impression that bass is lacking.

If possible I would suggest using one speaker for an 8 Ohm load.

And also I would lower your expectations regarding bass output very significantly, IF the cab is open back.
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Old 15th August 2018, 04:26 PM   #9
ITPhoenix is offline ITPhoenix  United States
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Good thought. The unit was built when everyone knew they were going down the pipe. There is no reason not to suspect they just threw in the 4 Ohm either due to supplier problems or to correct some user dissatisfaction.

An 8 Ohm speaker I suspect would have less of an overall SPL when compared to a 4 Ohm on the same system?

Interestingly, the two smaller 8 in. L and R speakers bleed some bass. I suspected an out-of-phase condition was attenuating to some degree, but disconnecting them made no difference.
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Last edited by ITPhoenix; 15th August 2018 at 04:42 PM. Reason: added clarity
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Old 15th August 2018, 04:39 PM   #10
pblix is offline pblix  United States
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Perhaps I'm expecting too much. There is no cabinet--it's open air.
Seems to me that's your problem right there.
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